1. Field of the Invention
The present invention relates to a magnetic drive apparatus for use in, for example, a speaker, the magnetic drive apparatus including a coil wound around a bobbin and a member for generating a magnetic field across the coil. The invention also relates to a method for manufacturing the coil of the magnetic drive apparatus.
2. Description of the Related Art
FIG. 5 is a sectional view showing one-half of a speaker installed in, for example, a vehicle. FIG. 6A is a front view showing a coil (voice coil) C which forms a portion of a magnetic drive apparatus A of the speaker. FIG. 6A also shows a bobbin 3 around which the coil C is wound. FIG. 6B is a top view of the coil C and the bobbin 3 shown in FIG. 6A.
The portion of the magnetic drive apparatus shown in FIG. 6A includes the tubular bobbin 3 which is made from a paper material or a resin-impregnated paper material. The coil C is formed by winding a covered lead wire 4 (round copper wire or flat wire) around the outer peripheral surface of the bobbin 3. The coil C includes a first coil C1 and a second coil C2, the coils C1 and C2 being spaced apart by a distance d along the axis of the bobbin 3. The coils C1 and C2 are wound in opposite directions around the axis of the bobbin 3.
A method will now be explained for winding the lead wire 4 around the bobbin 3, i.e., a method for manufacturing the coil C of the magnetic drive apparatus A.
In the method for winding the wire 4 around the bobbin 3, a spacer S1, which is shown in FIG. 7, is used as an auxiliary member. The spacer S1 is formed of a material which can be slightly deformed, such as a plastic material. The spacer S1 is formed in a ring-like shape having a thickness d measured in an axial direction of the spacer S1. The spacer S1 includes a hole 22 having internal diameter r which is equivalent to or slightly smaller than an outer diameter R of the bobbin 3 (see FIG. 6A). Edge portions 21a and 21b, which opposedly face each other across a wire-passing portion 21, are formed to have a planar shape which extends along the axial direction (the vertical direction in FIG. 7). The wire passing portion 21 is formed by removing a section of the ring-shaped material forming the spacer S1.
As illustrated in FIG. 6B, the hole 22 of the spacer S1 fits around the outer peripheral surface of the bobbin 3. A piece of wire 4 is then wound around the bobbin 3 adjacent the top surface 23a of the spacer S1. The first coil C1 is thus formed. Similarly, the wire 4 is wound around the bobbin 3 adjacent a bottom surface 23b of the spacer S1, thus forming the second coil C2.
More specifically, as shown in FIG. 6A, starting from a first leading portion 1a of the coil, the wire (lead wire) 4 is wound around the outer peripheral surface of the bobbin 3, for example, in the direction .alpha., to form at least one loop, thus forming an inner layer of the first coil C1. At an end portion 1b of the first coil C1, the wire 4 is bent substantially perpendicular at the upper corner of the edge portion 21a of the spacer S1 such that the wire 4 extends linearly downward along the planar surface of the edge portion 21a, and is finally bent perpendicularly at the lower corner of the edge portion 21a to form a leading portion 1c of the second coil C2. The second coil C2 is wound, for example, in the direction .beta., which is opposite to the direction in which the first coil C1 is wound. After the wire 4 is wound at a plurality of turns to form the second coil C2, at an end portion 1d of the second coil C2 is bent substantially perpendicular at the lower corner of the edge portion 21b of the spacer S1 such that the wire 4 extends linearly upward along the planar edge portion 21b. The wire 4 is further bent substantially perpendicularly at the upper corner of edge portion 21b at a leading portion 1e and is wound in the direction .alpha. on the inner layer of the coil C1 which has already been formed. When the total number of turns of the first coil C1 is equal to the total number of turns of the second coil C2, the wire 4 is bent upward at an end portion 1f.
After the coils C1 and C2 are formed, the spacer S1 is detached from the outer peripheral surface of the bobbin 3. The wire 4 forming the first and second coils C1 and C2 is fixed to the bobbin 3 either by an adhesive, or by a paper material wound around the outer surfaces of the first and second coils C1 and C2. Formation of the coil C is thereby completed.
The speaker shown in FIG. 5 includes a sound-producing cone (diaphragm) 12 mounted within a frame 11. An opening formed in an inner portion 12b of the cone 12 is covered with a domed section 13, while an outer edge 12a is connected to an opened end 11a of the frame 11 by a deformed suspension portion 14 that has a curved semi-cylindrical shape. The edge of the inner portion 12b of the cone 12 is supported by the frame 11 using a damper 15. The damper 15, which is formed of, for example, a resin-fiber-braided flexible sheet, a paper material or a resin film, is constructed in the form of a plurality of concentric waves. The cone 12 is vibratably supported on the frame 11 by the above-described suspension portion 14 and the damper
The bobbin 3 is attached to the inner portion 12b of the cone 12. A magnetic-field generating member is disposed at the base portion within the frame 11. The magnetic-field generating member includes a magnet 18 and a yoke 17 formed of a highly-permeable material, both components being fixed to the base portion of the frame 11. Gap G1 is formed between the N-pole surface of the magnet 18 and the yoke 17, while gap G2 is formed between the S-pole surface and the yoke 17. The above-described first coil C1 is located within gap G1, while the second coil C2 is positioned within gap G2. A voice current is passed through the wire (lead wire) 4 so as to flow in the first and second coils C1 and C2 in the opposite directions. The bobbin 3 and the cone 12 are vibrated in response to the above-described voice current and magnetic fields generated across the respective first and second coils C1 and C2 located between the magnet 18 and the yoke 17.
In the coil C provided for the above-described magnetic drive apparatus A of the speaker, the first and second coils C1 and C2 are separated by the distance d measured along the axis of the bobbin 3. Connecting portions 4a and 4b of the wire 4 are connected between the first and second coils C1 and C2. The connecting portion 4a is bent perpendicularly at the end portion 1b and the leading portion 1c adjacent the top and bottom corners, respectively, of the edge portion 21a of the spacer S1, as shown in FIG. 6A, thereby disadvantageously connecting the coils C1 and C2 linearly along the planar surface of the edge portion 21a. Similarly, the connecting portion 4b is also bent perpendicularly at the end portion 1d and the leading portion 1e because of the configuration of the edge portion 21b of the spacer S1, thus linearly connecting the coils C1 and C2.
Wire 4 is bent perpendicularly at the end portions 1b and 1d and the leading portions 1c and 1e in the manner described above, causing the formation of bent corners. Thus, there is an increase in stress in the wire 4 at the bent corners and also a rise in resistance. Consequently, if a high-output voice current is allowed to flow in the coil C, a wire break may occur because of the heat generated at the bent corners. Also, in the manufacturing method, the wire 4 is bent perpendicularly at the top and bottom corners of the edge portions 21a and 21b of the spacer S1, as shown in FIG. 6A, thus easily causing damage to a coating on the wire at the bent corners and further bringing about an insulation fault. If the bent corners are sharp, more serious damage may be caused, that is, the wire 4 may be broken while it is wound to form a coil.